Default value not available in response

Here is another patch which also sets the default value to "false" for checkbox elements when the default value is not set. This seems to work better with react-jsonschema-form and conditions.

This patch sets the default value to empty string "" which works better with conditions and seems to also work with numeric fields.

Apparently the empty default values just mess with form validation, so the first patch should be enough. Marking this RTBC.
I'm not sure how this should be done, so I'll just upload the first patch again in this comment.

Changing the patch author to Nathan B.